We assume the atomic electron moves in a spherically symmetric potential
\(V(r)=e A^{(\mathrm {ext})}(r)\) , for which the Dirac equation has eigenfunctions
of the form [1]}
\(\phi _{\kappa , \mu ,E}(r, \theta , \varphi ) = \frac{1}{r} \left(\begin{array}{l}g_{\kappa ,E}(r) \chi ^{\mu }_{\kappa }(\Omega ) \\i f_{\kappa , E} (r) \chi ^{\mu }_{-\kappa }(\Omega )\end{array}\right).\) 
